G4 Dwell: Pausing a CNC Program the Right Way

G4 holds axis motion for a fixed time while everything else — spindle, coolant, modal state — keeps running. It's a non-modal, one-shot command: one block, one pause, done.

S12000 M3
G4 P2        ; wait 2 s for spin-up (LinuxCNC/GRBL: P in SECONDS)
G0 X0 Y0 Z5
G1 Z-3 F200

#The P word trap

The single thing to know about G4 is that the unit of P changes per control, and getting it wrong multiplies your pause by 1000:

A FANUC-style G4 P500 pasted into a GRBL file turns a half-second chip break into a coffee break; the reverse turns a deliberate 2-second spin-up into 2 ms of nothing. FANUC also accepts X or U with a decimal for seconds (G4 X1.5), and lathes can dwell in spindle revolutions — check the specific manual when it matters.

When porting between controls, dwell lines are one of the places a find-and-check pass is mandatory — they're short, rare, and silently wrong.

#Legitimate dwells

• Spindle spin-up where the control doesn't wait for at-speed (GRBL posts insert one automatically after M3).
• Bottom of a hole: a revolution or two at depth cleans up a counterbore floor — though G82 builds exactly this into the drilling cycle, which is the better spelling.
• Process settling: coolant reaching the cut, an air blast clearing a pocket, a mister priming.
• Corner sharpening on controls without exact-stop: a tiny dwell lets the machine catch up before direction reverses. (Modern path-blending controls have better tools — G61/G64.)

#Common mistakes

• Wrong unit for the target control. The whole table above. It's the most common porting bug after unit mode itself.
• Dwelling instead of stopping. A 30-second dwell "to check the part" is a race against the program. Inspection holds are what M0/M1 are for — they wait for you.
• Dwells as a chatter fix. If a cut needs pauses to survive, the feeds/speeds are the problem; the dwell just spreads the damage out.
• Expecting G4 to hold spindle state changes. M5 G4 P1 M3 does not guarantee a stopped-then-restarted spindle reaches speed — the dwell runs concurrently with the spindle's own ramp on some controls. Pair state changes with the next dwell, not the same block.